Britain said on Thursday there was a significant possibility that Islamic State's Egyptian affiliate was behind a suspected bomb attack on a Russian airliner that killed 224 people in the Sinai Peninsula.
Russia dismissed the claim as speculation and Egypt said there was no indication so far that a bomb was to blame.
The topic is sensitive for Russia, whose warplanes have launched raids against Islamic State in Syria, and for Egypt, which depends heavily on revenues from tourism.
Asked if he thought Islamic State was responsible, Foreign Secretary Philip Hammond said: "ISIL-Sinai have claimed responsibility for bringing down the Russian aircraft, they did that straight away after the crash.
"We've looked at the whole information picture, including that claim, but of course lots of other bits of information as well, and concluded that there is a significant possibility," he said on Sky television.
British Prime Minister David Cameron said it was more likely than not that a bomb was to blame.
"We cannot be certain that the Russian airliner was brought down by a terrorist bomb, but it looks increasingly likely that that was the case," Cameron said.
U.S. and European security sources say evidence now suggests that a bomb planted by Islamic State's Egypt affiliate – Sinai Province – was the likely cause of the crash. The sources stressed they had reached no final conclusions about the crash.
Britain, Ireland and the Netherlands banned flights to and from Sharm al-Sheikh, where the doomed flight originated, while Germany urged travelers to avoid the Sinai Peninsula.
Dmitry Peskov, a spokesman for President Vladimir Putin, said Russian planes were still flying to and from Sharm al‐Sheikh.
"Theories about what happened and the causes of the incident can only be pronounced by the investigation," Peskov said.
"So far, we have heard nothing like this from the investigation. Any kind of similar assumptions like this are based on information that has not been checked or are speculation."